1. Field of the Invention
The present invention relates to a drive device aimed at a passive matrix type display panel in which for example organic EL (electroluminescent) elements are employed as light emitting elements, and particularly to a drive device and a drive method of a light emitting display panel which realizes current drive type gradation control including γ (luminosity factor) correction without increasing the circuit scale.
2. Description of the Related Art
A display panel constructed by arranging light emitting elements in a matrix pattern has been developed widely, and as the light emitting element employed in such a display panel, an organic EL element in which an organic material is employed in a light emitting layer has attracted attention. This is because of backgrounds one of which is that by employing, in the light emitting layer of the element, an organic compound which enables an excellent light emission characteristic to be expected, a high efficiency and a long life which can be equal to practical use have been advanced.
The organic EL element can be electrically replaced by a structure composed of a light emitting component having a diode characteristic and a parasitic capacitance component which is connected in parallel to this light emitting component, and it can be said that the organic EL element is a capacitive light emitting element. When a light emission drive voltage is applied to this organic EL element, at first, electrical charges corresponding to the electric capacity of this element flow into the electrode as displacement current and are accumulated. It can be considered that when the light emission drive voltage then exceeds a predetermined voltage (light emission threshold voltage=Vth) peculiar to this element, current begins to flow from one electrode (anode side of the diode component) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current.
Meanwhile, regarding the organic EL element, due to reasons that the voltage-intensity characteristic thereof is unstable with respect to temperature changes while the current-intensity characteristic thereof is stable with respect to temperature changes and that degradation of the organic EL element is considerable when the organic EL element receives excess current so that the light emission lifetime is shortened, and the like, a constant current drive is performed generally. A passive drive type display panel employing such organic EL elements has already been put into practical use partly.
As gradation control methods of the passive drive type display panel, time gradation method in which light emission time during each scan period is controlled to obtain a predetermined gradation and current gradation method in which drive current given to a light emitting element during each scan period is controlled to obtain a predetermined gradation have been proposed. Although either of the gradation control methods described above has advantages and shortcomings respectively, specifically the latter current gradation method has been said to be able to prolong the lifetime of the EL element generally compared to the case where the time gradation method is adopted. The reason is that while control in which an approximately maximum drive current flows is performed at a light emission time of the EL element in the case where the time gradation method is adopted, a chance that a maximum drive current flows rarely occurs in the case of the current gradation method.
Here, in the case where the latter current gradation method is adopted, it is relatively easy to linearly control the drive current value given to the light emitting element in response to a gradation. In this case, for example, a plurality of resistors which have the same resistance value are connected in series or the like to construct so-called ladder resistors so as to draw electrical potentials of respective connection points so that the drive current generated based on these potentials is supplied to the light emitting elements.
However, in the case where current gradation including γ correction is to be realized with the above-described structure, the above-described relatively simple structure cannot satisfy it, and a problem that the circuit scale thereof is considerably large occurs for the following reasons.
That is, FIG. 1 shows one example of a γ correction curve which is suitably adopted in the case where this type of EL element is employed, where the horizontal axis represents gradation and the vertical axis represents light emission intensity. That having been said, an intensity characteristic with respect to ideal gradation in the case where the EL element is employed is based on a correction curve of the order of γ=1.8–2.0.
As understood from the γ correction curve exemplified in this FIG. 1, it is necessary to allow light emission intensity differences for each gradation to be considerably small in a low gradation side and to allow intensity differences for each gradation to be large in a high gradation side. Accordingly, since it is necessary to finely control light emission intensity differences for each gradation particularly in a low gradation side, in the case where this is to be realized in the structure of the aforementioned ladder resistors, it becomes necessary to device means in which a large number of resistors which have slightly different resistance values are prepared so that these resistors, for example, are serial-parallel combined or the like. In that case, a case where switching transistors or the like for performing switching control for the serial-parallel connection relationship are needed may occur, and thus such a circuit structure has to be complex and large-scaled. Further, in the case where a user needs to change a γ correction characteristic, the respective resistors have to be variable.
In order to avoid the above-described problems, a means may be considered wherein the ladder resistors are made relatively simple and a DAC (digital to analog converter) prepared to extract voltage outputs from the ladder resistors is allowed to have the above-described γ correction characteristic. However, in the case where such a means is employed, another problem that the control bit number of the DAC has to be large occurs.
Japanese Patent Application Laid-Open No. 2003-288051 discloses that a current mirror circuit which generates drive current values given to light emitting elements based on the output of the DAC is allowed to have the function of the above-mentioned γ correction characteristic, preventing the ladder resistor combination as described above or the DAC from having a γ correction means.
In a γ correction circuit disclosed in Japanese Patent Application Laid-Open No. 2003-288051, a load resistor of the current mirror circuit is allowed to be variable so that the drive current given to light emitting elements is controlled, thereby providing a γ characteristic. However, referring to the specific γ correction means disclosed in Japanese Patent Application Laid-Open No. 2003-288051, such a means employs a large number of resistors (ladder resistors) and a DAC controlled by several bits are employed together in order to vary the load resistor of the current mirror circuit, and the basic structure thereof does not substantially differ from the one in which the ladder resistors and the DAC are combined.
In the case where a user needs to change a γ correction characteristic, even the structure disclosed in Japanese Patent Application Laid-Open No. 2003-288051 has a problem that a variable resistor or the like has to be prepared separately, and this problem is similar to that of the above-described conventional example.